Raman effect is one of the most widely used phenomena in chemical spectroscopy. Over the past 80 years, this effect has been measured by irradiating a sample with intense monochromatic light and detecting the minute amount (one part in 109) of frequency shifted scattered light. A typical Raman setup utilizes a high rejection, low insertion loss, long pass filter to reject the incident pump light. This is followed by a sensitive detector coupled to a high resolution spectrometer to record the molecular vibrational spectrum. By scanning the light beam or the sample while the Raman effect is being measured, a two-dimensional (2D) map of Raman signal can be created. However, since the effect is measured with light, the spatial resolution of Raman-based microscopy is limited by the wavelength of the light being used.
In order to enhance the spatial resolution of Raman microscopy, Raman measurement can be combined with atomic force microscope to yield a technique known as tip-enhanced Raman spectroscopy (TERS). In TERS, a metallic atomic force microscopy (AFM) tip is used to generate an enhanced near-field light, which scatters off a sample in the immediate vicinity of the tip. The scattered light is then analyzed by using far-field optics to measure the Raman effect originating from within the scattering volume. With TERS, Raman effect originating from a single molecule and spatial resolution on the order of 10 nm have been reported. However, the quality of data acquired by TERS depends very sensitively on the tip shape, in a manner that is not well controlled. In addition, the far-field background signal is much greater and interferes with the minute signal from the near-field scattering. As a result, TERS is far from being widely adopted despite its potential usefulness.